1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus such as an optical disc device, and more particularly, to an optical information recording/reproducing apparatus for recording or reproducing information on a recording layer of an optical recording medium using a solid immersion lens (hereinafter referred to as “SIL”).
2. Description of the Related Art
In general, in order to improve the recording density of an optical disc, a demand is made to shorten the wavelength of a light used in recording and reproduction, increase a numeric aperture (NA) of an objective lens, and decrease an optical spot diameter on an optical disc recording surface.
As one method of meeting the above-mentioned demand, an attempt has been made in which a front lens of objective lenses comes close to the recording surface to the degree of a fraction of divers of the recording wavelength to constitute a so-called SIL. Thus, an attempt has been made to set NA to 1 or more even in the air.
Those techniques are disclosed in more detail in, for example, Japan journal applied physics, vol. 44 (2005), pp. 3564 to 3567, “Near field recording on first-surface write-once media with a NA=1.9 solid immersion lens”. Further, the techniques are disclosed in more detail in Optical data storage 2004, proceedings of SPIE vol. 5380 (2004), “Near field read-out of first-surface disk with NA=1.9 and a proposal for a cover-layer incident, dual-layer near field system”.
The related art will be described with reference to FIGS. 19 to 23. First, a description will be given of the configuration of an optical pickup for a near field recording that is disclosed in Japan journal applied physics, vol. 44 (2005), pp. 3564 to 3567, with reference to FIG. 19. Referring to FIG. 19, reference numeral 1 denotes a semiconductor laser, 2 is a collimator lens, 3 is a beam shaping prism, 4 is an NBS, 5, 15, 19 and 26 are lenses, 6 is an LPC-PD, 7 is a PBS, 8 is a QWP, 9 is an expander lens, 10 is an objective lens, 11 is an SIL, 12 is a dual-layer disc, 13 is an HWP, 14 is a PBS, 16 is a PD1, 17 is an RF output, 18 is an NBS, 20 is a PD2, 21 is a tracking error, 27 is a PD3, and 28 is a gap error.
Referring to FIG. 19, a light flux that is output from a semiconductor laser 1 having a wavelength of 405 nm is converted into a collimated light flux by a collimator lens 2, and then input to a beam shaping prism 3 to provide an isotropic light quantity distribution. Further, the light flux that is transmitted by the polarization beam splitter (PBS) 7 through a non-polarization beam splitter (NBS) 4 passes through a ¼ wavelength plate (QWP) 8, and is converted into a circularly polarized light from a linearly polarized light. The photodetector (LPC-PD) 6 receives the light flux that has been reflected by the non-polarization beam splitter (NBS) 4, and controls the output power of the semiconductor laser 1.
The light flux that has been transmitted by the ¼ wavelength plate 8 is input to the expander lens 9. The expander lens 9 corrects spherical aberration that occurs in an objective lens or an SIL which will be described later. The expander lens 9 is so adapted as to control an interval between two lenses according to the spherical aberration. The light flux from the expander lens 9 is input to the objective lens (rear lens 10).
An objective lens unit includes the objective lens (rear lens) 10 and the SIL (front lens) 11. The objective lens 10 and the SIL (front lens) 11 are held by a lens holder as will be described later. The lens holder is mounted on a 2-axis actuator (not shown) that drives two lenses integrally in a focus direction and in a tracking direction.
The SIL 11 is of two types shown in FIGS. 20 and 21. In FIG. 20, a light flux that has been focused by an objective lens (rear lens) 101 is collected on a bottom surface of a hemispherical lens SIL 102-a. The light flux is input perpendicularly to the spherical surface of the hemispherical lens, and then collected on the bottom surface through the same optical path as that in the case where there is no hemispherical lens. As a result, the wavelength is equivalently shortened by the refractive index of the hemispherical lens, and therefore the optical spot diameter is reduced.
That is, when it is assumed that the refractive index of the hemispherical lens is N, and the numeric aperture of the objective lens 101 is NA, a light spot corresponding to N×NA is obtained on the recording surface of the optical disc 103. For example, when the objective lens 101 of NA=0.7 is combined with the hemispherical lens SIL-102a, NAeff=1.4 is achieved when it is assumed that the effective NA is NAeff. An error of about 10 μm in the thickness of the hemispherical lens 102-a can be allowed, so the mass production is facilitated.
On the other hand, in FIG. 21, a light flux that has been focused by the objective lens (rear lens) 101 is collected on a bottom surface of an super-hemispherical lens SIL 102-b. The bottom surface is spaced apart from the center of the super-hemispherical lens SIL 102-b by R/N. When it is assumed that an angle defined between the optical axis and the light flux on the bottom surface is θt, the angle θt and an angle θi defined between the light flux that is input to the super-hemispherical lens SIL 102-b and the optical axis meet a relationship of Expression (1).sin θt=N×sin θi  (1)
Since sin θi is NA of the objective lens 101, the light spot corresponding to N2×NA is obtained on the recording surface of the optical disc 103, taking the fact that the light flux is collected in the SIL of the refractive index N into account. The NA of the objective lens 101 is limited to 1/N or lower through Expression (1) under the condition where the light flux can be input to SIL 102-b. 
When super-hemispherical lens SIL 102-b is made of a material for glass lens of N=2, even if the objective lens 11 is formed of an objective lens of a relatively low NA, for example, NA=0.5, it is possible to obtain the light spot corresponding to NAeff=2.0. However, there arises a drawback that an error in the thickness of the super-hemispherical lens 102-b cannot be allowed to be higher than about 1 μm.
In any SIL, only in the case where the distance between the bottom surface of the SIL and the optical disc 103 is a short distance that is equal to or less than a fraction of divers of the wavelength 405 nm of the light source, for example, 100 nm or less, the light spot affects the recording surface from the bottom of SIL as an evanescent light, and recording/reproduction can be conducted by the light spot diameter of NAeff. In order to keep the above-mentioned distance, a gap servo is employed. The optical disc 12 of FIG. 19 is formed of a dual-layer disc having two recording layers which will be described later with reference to FIGS. 22 and 23.
Returning again to FIG. 19, an optical system of a return path will be described. The light flux that has been reflected by the dual-layer disc 12 becomes a circularly polarized light that is inversely rotated, and is input to the SIL 11 and the objective lens 10, and again converted into a collimated light flux. In addition, the light flux that has passed through the expander lens 9 and the ¼ wavelength plate 8 and has been linearly polarized in a direction orthogonal to the going path is reflected by the PBS 7, and then input to the ½ wavelength plate (HWP) 13.
On the other hand, an S polarized light component in the light flux whose polarization plane is rotated by 45° by the ½ wavelength plate (HWP) 13 is reflected by the polarization beam splitter 14, and then collected on the photodetector (PD1) 16 through the lens 15. Information on the optical disc 12 is reproduced from the RF output 17 of the photodetector (PD1) 16.
Further, a P polarized light component in the light flux whose polarization plane is rotated by 45° by the ½ wavelength plate (HWP) 13 is transmitted by the polarization beam splitter 14, reflected by the non-polarization beam splitter (NBS) 18, and then collected on the two-division photodetector (PD2) 20 through the lens 19 A tracking error 21 is obtained from an output signal of the two-division photodetector (PD2) 20.
On the other hand, the light flux of NAeff<1 that does not conduct total reflection among the light flux that is reflected by the bottom surface of SIL 11 is reflected as the circularly polarized light that rotates inversely to the input light as with the reflected light from the double-layer optical disc 12. The light flux of NAeff≧1 which occurs total reflection produces a phase difference δ which is represented by the following expression (2) between the P polarized light component and the S polarized light component, and forms an elliptically polarized light that is deviated from the circularly polarized light.tan(θ/2)=cos θi×√(N2×sin2 θi−1)/(N×sin2 θi)  (2)
Accordingly, when the light flux passes through the ¼ wavelength plate 8, the light flux includes the polarized light component in the same direction as the going path. The polarized light component is transmitted by the PBS 7, reflected by the NBS 4, and collected on the photodetector (PD3) 27 through the lens 26. The amount of light flux is monotonically more reduced as the distance between the bottom surface of the SIL and the dual-layer disc is shorter in the near field region, and therefore the polarized light component can be used as the gap error signal 28.
When a target threshold value is determined in advance, the gap servo is conducted such that the distance between the bottom surface of the SIL and the optical disc can be held to a desired distance of 100 nm or less. The gap servo is disclosed in detail in the above-mentioned article of Japan journal applied physics, vol. 44 (2005), pp. 3564 to 3567. Further, since the light flux is not modulated by the recording information on the optical disc 12, a stable gap error signal can be obtained regardless of the presence or absence of the recording information.
As described above, the super-hemispherical SIL has the advantage that NA can be easily increased. For example, when NAeff=2, recording of 150 GB can be conducted on the disc that is 120 mm in diameter. However, it is necessary to extremely severely control a manufacture error in the thickness of the lens SIL. Further, since the evanescent light does not reach the recording layer unless the refractive index of a protective layer that protects the recording layer is higher than NAeff, the material of the protective layer must be necessarily made of an inorganic material whose refractive index exceeds 2.
That is, in the super-hemispherical SIL, the protective layer made of the organic material, in which the protective layer can be coated by spin coat inexpensively but which is low in the refractive index (N=about 1.6), cannot be used. Since the protective layer that prevents the recording layer from being damaged by an abrasion requires at least about several μm, production of the protective layer by using the inorganic material requires high costs. Likewise, in the case of using the super-hemispherical SIL, it is difficult to provide a multi-layer structure that laminates plural recording layers on each other with an intermediate layer made of the organic material.
On the other hand, the spherical SIL has a limit of NAeff=about 1.5 from the viewpoint of the NA of the objective lens that can be used inexpensively. In this case, recording of 84 GB can be conducted on the disc that is 120 mm in diameter. However, since the refractive index of the protective layer that protects the recording layer can be selected to be about 1.6, it is possible to use the protective layer made of an inexpensive organic material.
Likewise, in the case of using the hemispherical SIL, it is possible to provide a multi-layer structure that laminates plural recording layers on each other with the intermediate layer made of the organic material. For example, the dual-layer disc is 168 GB in the recording capacity which is superior to a case using the super-hemispherical SIL of NAeff=2. In addition, a manufacture error of the hemispherical SIL is relatively so lax as to allow mass production. The comparison of those SIL is disclosed in detail in the above-mentioned article of Optical data storage 2004, proceedings of SPIE vol. 5380 (2004).
Now, a description will be given of the details of the dual-layer disc 12 and the hemispherical SIL with reference to FIGS. 22 and 23. In the drawings, the same symbols denote identical members. The dual-layer disc 12 has an L0 recording layer 12-2 disposed on a polycarbonate substrate 12-1. The L0 recording layer 12-2 has an information track and a track along which pits are defined.
On the L0 recording layer is disposed, for example, an L1 recording layer 12-4 having an information track and a track along which pits are formed through, for example, an intermediate layer 12-3 having a constant thickness of 3 μm which is made of 2P (photo polymer), likewise. In addition, on the L1 recording layer 12-4 is disposed, for example, a cover layer 12-5 having a constant thickness of 3 μm which is made of 2P (photo polymer).
The center of a sphere of a virtual hemispherical SIL 11 (the center of a circle indicated by a dotted line) is located substantially at the intermediate between the L0 recording layer 12-2 and the L1 recording layer 12-4. In the case where the light flux is focused on the L0 recording layer 12-2, an interval between the objective lens 10 and the SIL 11 is adjusted to d1 by a voice coil motor 201 as shown in FIG. 22. The light flux that has been collimated by the expander lens 9 passes through the objective lens 10 and the SIL 11, and is focused on the L0 recording layer that is located at a position slightly farther from the SIL than the virtual center of the above-mentioned sphere.
Further, in the case where the light flux is focused on the L1 recording layer 12-4, the interval between the objective lens 10 and the SIL 11 is adjusted to d2 (d2>d1) by the voice coil motor 201, as shown in FIG. 23. The light flux that has been collimated by the expander lens 9 passes through the objective lens 10 and the SIL 11, and is focused on the L1 recording layer 12-4 that is located at a position slightly closer from the SIL than the virtual center of the above-mentioned sphere.
Jump of the interlayer between the L0 recording layer and the L1 recording layer of the optical disc is conducted by controlling the objective lens 10 by the voice coil motor 201 and adjusting the interval between the objective lens 10 and the SIL 11, as shown in FIG. 23. This technique is disclosed in detail in the above-mentioned article of Optical data storage 2004, proceedings of SPIE vol. 5380 (2004).
The voice coil motor 201 that adjusts the interval between the objective lens 10 and the lens SIL 11 is mounted on a lens holder 202. The lens holder 202 is controlled such that the distance between the SIL 11 and the disc 12 is kept to a given value by a 2-axis actuator (not shown) according to the gap error signal 28. Further, the lens holder 202 is controlled such that the light spot tracks a desired track by the 2-axis actuator according to a tracking error signal 21.
The conventional optical information recording/reproducing apparatus for near field recording using the hemispherical SIL 11 and the dual-layer disc 12 suffers from the following problems. That is, the distance between the SIL and the disc is merely kept to a desired value according to the gap error signal. Therefore, in order to focus the light flux on the L0 layer or the L1 layer with precision, the focus error signal cannot be used, and it is necessary to always monitor the amplitude or the modulation degree of the tracking error signal or the RF signal. This is because the reflected light from the bottom surface of the SIL 11 is mixed with the focus error signal as a noise as described above.
Therefore, even though a slight thickness unevenness occurs in a cover layer or an intermediate layer of the disc, the light spot is incapable of tracking the unevenness rapidly, thereby making it difficult to record or reproduce information with precision. Further, even if the wavelength of the semiconductor laser changes due to a temperature change, the light flux is incapable of tracking the change of the wavelength quickly, thereby making it difficult to record or reproduce information with precision.
Further, in conducting interlayer jump, because the focus error signal cannot be referred to, it is difficult to conduct quick focus jump, and jump frequently fails.